Runoff Impacts and LID Techniques for Mansionization-Based Stormwater Effects in Fairfax County, Virginia

Abstract

This case study uses the Natural Resources Conservation Service (NRCS) curve number method to quantify the increase in stormwater runoff volume from infill residential redevelopment, or mansionization, in a 0.13-km2 residential subwatershed of Fairfax County, Virginia. Analysis of 10 redeveloped lots in the subwatershed showed an average increase in impervious cover from 8 to 28% per parcel after redevelopment, resulting in an average increase in runoff volume of 18% per parcel for the 10-year, 24-h storm. From 1997 to 2009, the total impervious cover in the subwatershed increased from 18 to 25%, resulting in a calculated 5.6% total increase in runoff volume. Low-impact development (LID) techniques were modeled as retrofits in the subwatershed to mitigate the increase in runoff volume. Measures modeled include bioretention basins, infiltration trenches, amended soils, permeable pavement, and cisterns. Results indicate that placing bioretention basins or infiltration trenches on 0.5% of the subwatershed or amending 20% of the open space with soil composts would reduce the runoff volume back to the 1997 quantity for the 1-year, 24-h storm.